The
parts of a crankshaft may be identified by various words. However, the terms in
figure 4-19 are the ones that are most commonly used in the NAVSEA technical
manuals for the engines used by the Navy.

The
MAIN JOURNALS serve as the points of support and as the center of rotation for
the shaft. As bearing surfaces, the main journals and the connecting rod
journals of crankshafts are surface-hardened so that a longer wearing, more
durable bearing metal can be used without causing excessive wear of the shaft.

As
illustrated in figure 4-19, crankshafts have a main journal at each end of the
shaft with an intermediate main journal between the cranks. Each CRANK (throw)
of a shaft consists of three parts, two webs and a pin, as shown in figure
4-19. Crank webs are sometimes called cheeks or arms. The cranks, or throws,
provide points of attach-ment for the connecting rods, which are offset from
the main journals.

In
many crankshafts, especially in large engines, the connecting rod journals and
main journals are of hollow construction. Hollow construction not only reduces
weight considerably but also increases torque capability of the crankshaft and
provides a passage for the flow of lubricating oil (fig. 4-20).

The
forces that turn the crankshaft of a diesel engine are produced and transmitted
to the crankshaft in a pulsating manner. These pulsa-tions create torsional
vibrations, which are capable of severly damaging an engine if they are not
reduced, or dampened, by opposing forces. Many engines require an extra
dampening effect to ensure satisfactory operation. It is provided by a
torsional vibration damper mounted on the free end of the crankshaft. Several
types of torsional dampers are currently in use.

On
some crankshafts, part of the web of the crankshaft extends beyond the main
journal to

form
or support counterweights. These counter-weights may be integral parts of the
web (fig. 4-19) or may be separate units attached to the web by studs and nuts,
or setscrews (fig. 4-21). Counterweights balance the offcenter weight of the
individual crank throws and thereby compensate for centrifugal force generated
by each rotating crank throw. Without such balance, the crank action will create
severe vibrations, particularly at the higher speeds. If such vibrations are
not controlled, the shaft would become damaged. Excessive vibration may lead to
complete failure of the engine. Counterweights use inertia to reduce the
pulsating effect of power impulses in the same manner as the flywheel.
Flywheels are described later in this chapter.